As the New England climate becomes warmer and wetter, the deer tick has seen its habitat and host population grow.
The deer tick, whose preferred host is the white-tailed deer, is the primary vector of Lyme disease, an infection that can affect any organ from the brain to the heart, resulting in fever, facial paralysis, arthritis, and other symptoms. The disease has seen a steady increase in reported cases over the last decade according to data collected by both the Center for Disease Control and Prevention and the Massachusetts Department of Health.
The rise in the number of Lyme cases has experts concerned.
“We have a chronic problem with Lyme disease. It’s the top identifiable vector-borne disease in the country. Period,” says Susan Elias, a vector-borne disease ecologist at the Maine Medical Center Research Institute. “It’s the second highest reportable infectious disease after chlamydia. It’s a big public health issue.”
Between 2005 and 2014, the state of Maine reported an increase in Lyme disease incidents to the CDC every year but one. Massachusetts has shown a less linear trend, but the state has still experienced an overall increase. With over 3,000 cases annually, the state reported the second-most cases in the country in each of the last three years. Between 2005 and 2011, the state had passed the 3,000 case mark only once.
A number of factors have resulted in this trend, Elias says. Large populations of the white-tailed deer, spreading invasive plant species like the Japanese barberry, and climate change are three of the biggest influences.
Protected by invasive plant species
A 2016 study published in Trends in Parasitology found that ticks and the diseases they spread are more likely to be influenced by long-term changes in climate than mosquitoes due to the tick’s multi-year life cycle. While mosquitoes with a much shorter life cycle can more easily adapt to short-term weather variations, the rising temperatures in New England favor the geographic spread of tick populations.
The summer months of June and July feature significantly higher incident rates of Lyme disease than the other months. This is a result of these summer months leading to more time outdoors for people in New England and a healthy environment for tick survival. Overall, rising temperatures from climate change are resulting in a greater number of weeks during the summer and early fall that have high Lyme disease potential.
Yet at the same time, the weather variability caused by climate change also poses a threat to ticks. Given their sensitivity to temperature and humidity, ticks need to find habitat that protects them from the more frequent extremes that climate change brings.
In this case, newly arrived invasive species of plants that dominate New England’s forest floors including the Japanese barberry, Japanese honeysuckle, and oriental bittersweet are providing the protective habitat that ticks need to survive climatic extremes. As a consequence, ticks are benefiting from the long term rise in temperatures brought by climate change, and surviving the shorter term temperature swings by way of these new invasive species.
“It protects a nymph, the little delicate nymph,” Elias says of these invasive species. “They’ll find a cooler microclimate in that barbary, so the survival rate is higher.”
Culling deer and limiting invasive plant species
With Lyme disease already the most common vector-borne disease in the country and climate change and the spread of invasive species creating an even more favorable environment for ticks, the question of how to prevent it becomes all the more important.
Due to the wide range of factors influencing the spread of the tick population, an integrated pest management, or IPM, strategy appears to be necessary. An IPM is a broad, multi-faceted approach to preserving the ecosystem from pests that are threatening it.
With respect to ticks specifically, Elias suggests a number of different strategies for curtailing the population and the spread of Lyme disease.
“In the long run, we might try to cut greenhouse gases,” Elias says. “In the near term, what can you do at the municipal level? You can treat hosts. If you have an overabundance of deer, you can make a decision about whether you want a lower deer density per square mile.”
Elias suggests a population density of no more than 12 deer per square mile, far lower than the 80 per square mile that the Massachusetts Department of Energy and Environmental Affairs says can be found in eastern parts of the state. It is often difficult, however, for a community to agree to severely limit the wildlife population.
Clearing out invasive plant species is another recommended but difficult approach. Such a strategy would require significant budget allocation. Acaricides, substances that are poisonous to mites and ticks, could be sprayed in the backyards of homes, the most likely place to contract Lyme disease. The effects of these synthetic sprays on children, pets, and pollinators, though, raise further issues.
The potential strategies to curtail the deer tick population and the diseases they spread are numerous, but all require government funding, collaboration between institutions, and consultation of the public.
“We know how to kill ticks. We know how to do it,” Elias says, but “there’s no silver bullet.”
While diseases spread by mosquitoes have more potential for reaching epidemic levels due to the ability of mosquitoes to adapt to short term weather patterns, it is important not to overlook the implications of climate change on a growing and spreading deer tick population. Elias hopes the problem will be addressed before it becomes an even more serious problem.
“There are so many opposing views,” she says, “and sometimes it takes something approaching a public health emergency that really gets people to work together.”